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Old 12th January 2009, 02:36 PM   #1
robmil is offline robmil  United Kingdom
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Default PCB layout for phonostage

I'm in the process of laying out the phonoclone. Is it best to use a ground plane or star? If a ground plane where does the 0 volts connect on this plane?

thanking all in advance

rob
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Old 12th January 2009, 02:42 PM   #2
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I have used a groundplane on one board level (usually bottom) and attached it near the GND input of the power supply input. All ground returns are brought to this locus. Mark Brasfield suggested this in one of his application notes -- and it works.
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Old 12th January 2009, 03:06 PM   #3
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Well, if you're interested (or feel bored ) you can read a lot on proper grounding techniques.

My few cents are:

A ground plane is for the lazy engineer that doesn't care (or does not want to care) about where the return currents flow. So a ground plane allows them to find the path of minimum impedance. Drawback: adds capacitance (order of pF) to every circuit node.

If you know the ins and outs of the electrical circuit (currents) you can do a star ground which has also the nice advantage that you can use this pcb-side also for routing other signals.

Keep power and signal ground separate in any case. Keep digital and analog separate.

Have fun, Hannes
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Old 12th January 2009, 03:41 PM   #4
robmil is offline robmil  United Kingdom
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Hi...thanks for the replies.

If I go the ground plane route, by separating power and signal grounds, it that by physical distance?

I see on Jan dupont site and his PCBs he uses ground planes throughout with success. the input grounds are at the other end of the PCB to the power gound in some cases...is that what is meant by separation?

rob
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Old 12th January 2009, 04:37 PM   #5
h_a is offline h_a  Europe
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Quote:
If I go the ground plane route, by separating power and signal grounds, it that by physical distance?
In general this is not necessary. With separate I mean electrically separate. Connect each ground separately to your ground star, not on the pcb. This avoids voltage drops in the cabling to your pcb due to large currents being sent into power ground.

For a phono stage it won't hurt to keep an eye on physical distance as well; but I'm talking more about avoiding putting them next to each other than about keeping them on opposite sides of the board

Anyway the current demands in a phono stage are very small in general.

Have fun, Hannes
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Old 13th January 2009, 01:43 AM   #6
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Default Re: PCB layout for phonostage

Quote:
Originally posted by robmil
I'm in the process of laying out the phonoclone. Is it best to use a ground plane or star? If a ground plane where does the 0 volts connect on this plane?

Hi Rob,
I've been working on one that uses an AC wall adapter with the regulated supply on the board with the preamp and ran into some hum that I think I have now fixed. I'm not sure how yours will be but here's what I found I had to do. I separated the supply ground with a 10 ohm resistor (R10 in the pic below). This cut the hum to near nothing. I'm also looking for clues as to how to reduce this even further.
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Old 13th January 2009, 05:18 AM   #7
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Quote:
A ground plane is for the lazy engineer that doesn't care (or does not want to care) about where the return currents flow.
This has nothing to do being lazy!
Working with small signals that requires 60dB (1000 times) amplification or more, a ground plane is must.
And like jackinnj says, the ground plane should be connected to the PSU ground terminal.

Quote:
This avoids voltage drops in the cabling to your pcb due to large currents being sent into power ground.
Where do you find these voltage drops and large currents in a phono stage? Maybe you should concider a redesign to match voltage and current to the actual job...........
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Old 13th January 2009, 06:27 AM   #8
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Quote:
Originally posted by h_a
Drawback: adds capacitance (order of pF) to every circuit node.
One square centimeter area = 3 pF => a trace over groundplane = less.

34 pF/m with a 1 mm wide trace according to
this calculator

Fraction of a pF is never a problem in these non-high impedance circuits.

One thing to care about is the feedback path though.
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Old 13th January 2009, 11:56 AM   #9
robmil is offline robmil  United Kingdom
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well, I'm going with Jan's ideas about ground planes. I spoke to a designer at a known pro-mixer company yesterday and he said that I 'really' need a ground plane. With digital I would need 3-4 planes - 1 digital, 1 analog, 1 ground and 1 power or sometimes you can get away with 1 plane that carries analog and digital signals BUT these need to be physically separated.

So, I'm going ground plane....BUT how?

Does my input lead from the RCA go to the PCB? Do all grounds (RIAA, grounds from ICs, power) all go the the plane at their nearest point like you see on Philips CD players.
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Old 13th January 2009, 07:50 PM   #10
robmil is offline robmil  United Kingdom
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MJL - Is connecting a 10R resistor between PSU Gnd and Audio Gnd common practice as I've never seen it done before. Do you use a 5 watt resistor for this? If the resistor value rises (heat etc) what happens to your RIAA accuracy?

Rob.
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